The aim of this study was to develop a simple and robust methodology for the routine estimation of primary production in coastal water bodies, such as the sea loughs of Northern Ireland. Primary production estimates are key to assessing the trophic status and carrying capacity of water bodies. The traditional methods for deriving production, although sensitive and reliable, are time consuming and not suitable for routine monitoring programmes. High frequency sampling was carried out for two years (April 2006 - March 2008) in Carlingford Lough (NI), to run incubations to derive primary production and microplankton community respiration.
The 14C technique was chosen for estimating primary production due to its high sensitivity. A photosynthetron and short (2 hour) incubations were used to derive the photosynthetic parameters, from photosynthesis/irradiance curves. Photosynthetic parameters showed seasonal variability and were significantly correlated with some environmental variables (e.g. ammonium, incubation temperature).
Values of chlorophyll concentration, light attenuation coefficient (Kd) and photosynthetic parameters were used to derive estimates of gross daily column production, ranging between 3.2 and 1210 mg C m-2 d-1. The seasonal trend in gross production showed one peak in spring and one at the end of the summer. Chlorophyll standing stock explained 71% of the variability in daily production. This increased to 89% when irradiance during the sampling and Kd were included in the relationship.
A truncated Fourier series (TFS; Fig. 1) was fitted to daily estimates of column production and used to estimate annual production as 116 g C m-2 y-1 with 90% confidence interval of 98-141 g C m-2 y-1. Annual microplankton community respiration was estimated as 117 g C m-2 y-1 (90% confidence interval 105-134 g C m-2 y-1). It was concluded that within the Lough there were periods of net production but these episodes were not cyclical and that on an annual balance, Carlingford Lough was a heterotrophic system.
This study confirmed that chlorophyll standing stock can be used to derive estimates of daily gross production and that the TFS analysis is a useful method for estimating annual production and quantifying the associated error to provide confidence intervals that could be used to assess long-term changes. The method was applied to Belfast Lough and gave an estimate of annual production of 102 g C m-2 y-1 with 90% confidence interval of 74-169 g C m-2 y-1. This initial test suggests that the chlorophyll stock/production relationship and the TFS can be used in other coastal water bodies and merits further validation and testing.
Fig 1. – The TFS time series fit to log-transformed daily column production estimates (mg C m-2 d-1) from June 2006 to March 2008, for Carlingford Lough.
Acknowledgements
This study is described by Capuzzo (2011). The work was funded by a research studentship from the Lough Agency in Northern Ireland, and facilities were provided by the Agri-Food and Bioscience Institute (AFBI) in Belfast.
References
Capuzzo E. (2011). Measuring and modelling the primary production of a sea lough in Northern Ireland. Ph.D. thesis, Edinburgh Napier University.
